Resistance variable memory elements, which include chalcogenide-based programmable conductor elements, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical such device is disclosed, for example, in U.S. Pat. No. 6,849,868 to Campbell, which is incorporated by reference.
In a typical chalcogenide-based programmable conductor memory device, a conductive material, such as silver, is incorporated into a chalcogenide glass. The resistance of the chalcogenide glass can be programmed to stable higher resistance and lower resistance states. An unprogrammed chalcogenide-based programmable conductor memory device is normally in a higher resistance state. A write operation programs the chalcogenide-based programmable conductor memory device to a lower resistance state by applying a voltage potential across the chalcogenide glass. The chalcogenide-based programmable conductor memory device may then be read by applying a voltage pulse of a lesser magnitude than required to program it; the resistance across the memory device is then sensed as higher or lower to define the ON and OFF states.
The programmed lower resistance state of a chalcogenide-based programmable conductor memory device can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed. The chalcogenide-based programmable conductor memory device can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the device to the lower resistance state. Again, the higher resistance state is maintained in a semi- or non-volatile manner once the voltage potential is removed. In this way, such a device can function as a variable resistance memory having at least two resistance states, which can define two respective logic states, i.e., at least a bit of data.
One exemplary chalcogenide-based programmable conductor memory device uses a germanium selenide (i.e., GexSe100-x) chalcogenide glass as a backbone. The germanium selenide glass has, in the prior art, incorporated silver (Ag) and silver selenide (Ag2Se).
Previous work by the inventor, Kristy A. Campbell, has been directed to chalcogenide-based programmable conductor memory devices incorporating a silver-chalcogenide material as a layer of silver selenide (e.g., Ag2Se) or silver sulfide (e.g., Ag2S) in combination with a silver-metal layer and a chalcogenide glass layer. The silver-chalcogenide materials are suitable for assisting in the formation of a conducting channel through the chalcogenide glass layer for silver ions to move into to form a conductive pathway.
Tin (Sn) has a reduced thermal mobility in GexSe100-x compared to silver and the tin-chalcogenides are less toxic than the silver-chalcogenides, therefore tin-chalcogenides (e.g., SnSe) have also been found to be useful in chalcogenide-based programmable conductor memory devices to replace silver selenide. However, sputtering of tin selenide to form such devices has proven difficult due to the increased density of the sputtered layers. This increased density (e.g., ˜6 g/cm3 sputtered compared to ˜3 g/cm3 evaporated) can prevent the motion of silver ions into the chalcogenide glass, thereby preventing the memory device from functioning. Therefore, evaporative deposition techniques have been used to deposit such material, which is generally a less efficient, more costly, slower, and less controlled technique for deposition. However, evaporation deposition of tin selenide and silver also incorporates some oxygen into the resulting layer, which provides for the lower density and allows for more mobility of silver ions.